1. Field of the Invention
The invention relates to spread spectrum communication, particularly with respect to phase locked loop design for the Global Positioning System (GPS).
2. Description of the Prior Art
The GPS is a navigation system utilizing a plurality of satellites in diverse orbital positions. Each satellite transmits an L-band carrier bi-phase modulated by a spectrum spreading Pseudo Random Noise (PRN) code identifying the satellite and by a 50 baud navigation data message. In the present day system, the carrier frequency is 1575.42 MHz and the PRN code has a clock rate of 1.023 MHz and a code length of 1023 bits. The code repeats the predefined sequence thereof once each millisecond.
In order to receive the GPS signal structure, a GPS receiver generates an exact duplicate of the spreading code and aligns it in time to the receive code using correlation techniques. The receiver thus removes the spreading code from the carrier (unmodulates the carrier) leaving only the bi-phase 50 baud data message thereon. The data message is used by the GPS receiving equipment to solve the navigation problem.
In order to demodulate the data message, the GPS receiver must phase lock thereto and coherently detect each bit. Conventional phase locked loops cannot lock to a biphase modulated carrier since this type of modulation has no carrier content. Biphase modulated carriers are of the suppressed carrier type with all the carrier energy in the sidebands. Two prior art designs utilized to recover the data are the Costas Loop and the Squaring Loop. These loops are discussed in detail in Digital Communications by Satellite by James J. Spilker, Prentice-Hall, 1977, pages 297-305. Both the Costas Loop and the Squaring Loop are of complex design requiring large numbers of components. Thus, these prior art loops tend to h=expensive and unreliable. Traditionally, the Costas Loop is far more often utilized in the GPS technology than the Squaring Loop. A Squaring Loop can track a bi-phase modulated carrier because the squaring process modulates the carrier. Particularly, the squaring process generates harmonics of which the even numbered harmonics have no modulation thereon. After squaring, the GPS receiver phase locks to the second harmonic.